This invention relates to a digital local switching system for switching voice and data signals comprising circuit switching calls, packet switching calls, and digitized signalling information.
The International Telegraph and Telephone Consultative Committee (CCITT) specified the network for offering digital communication service and designated it as an "Integrated Service Digital Network (ISDN)". The CCITT submitted I. Series recommendation on the basic specifications toward the end of 1984. According to this basic recommendation, ISDN's user network interface determines the boundary conditions for effectively connecting user's terminals with the network for optimal communication. The network is constructed with two B channels of 64 kilobits per second (Kpbs) and one D channel of 16 (Kbps) (2B+D). This interface is referred to as the basic access interface which enables, for example, communication through a speech channel (B1) of 64 Kbps, a data channel (B2) of 64 Kbps, and a data channel (D) for low speed packets of 16 Kbps.
The ISDN's user network interface allows each call to select various different services for use. Demands for service to the network or demands for a call are expressed in a message called SETUP. The SETUP messages are packetized and transferred through the D channel from a user to the network. The SETUP message is written with (1) a call reference value for identifying the call, (2) a bearer capability for designating the transfer rate and mode of connection needed by the particular call, (3) the channel identification for identifying which of the two B channels or D channel should be used, and (4) an ISDN destination address of the other party for designating the telephone number thereof. The function to deal with the call controlling procedures with such messages is called "layer-3" which is referred to in sections I.450 and I.451 of the above-mentioned CCITT Recommendation I. Series. The procedure for correctly transferring the messages of the layer-3 is called "layer-2" which is mentioned under the title; "Link Access Procedure on the D channel (LAP-D)" in I.440 and I.441 of the CCITT Recommendation. The LAP-D has functions such as connecting/disconnecting to and from a data link, identifying frames, sequence control, error detection, error restoration, flow control, etc. The LAP-D is provided with a service access point identifier ("SAPI") for identifying the service access point between the layers 2 and 3.
Sections I.430 and I.431 of the CCITT Recommendation specify channel structure, conditions or activation and suspension, speed, electric voltage level, number and forms of connector pins, etc. of layer 1.
In practice, however, various problems should be solved in order to practically structure an ISDN based on the specifications decided as above. One of those problems is how to construct a local switching system. In an ISDN, various different services should be received through a common subscriber interface. Firstly, the switching system should accommodate the user network interface, as specified by the CCITT Recommendation. Secondly, the switching system should connect calls of various types which are incoming from and outgoing to the ISDN subscribers, for instance, calls having both attributes of a circuit switching call and a packet switching call. The packet calls are classified into two types in ISDN; i.e., those transferred via a D channel, and those transferred via a B channel. Thirdly, the ISDN switching system should handle both of them.
As almost all of the prior switching systems are made for telephones, even if they are digital switching systems, they should be added with an ISDN digital line interface on the side of users in order to connect ISDN subscribers. Even if these ISDN subscribers are successfully connected, the prior switching systems cannot support both the circuit and the packet switching. More particularly, since the prior telephone switching systems are made for circuit switching, it is impossible for them to handle packet switching which is fundamentally different from circuit switching, in the switching system.
In order to realize a packet switching service of ISDN subscribers, a prior local switching system should be either replaced with a digital telephone exchange with means for processing packet-switched data in addition to circuit switched data like the one disclosed in U.S. Pat. No. 4,486,878; or it should be provided separately with a switching system dedicated exclusively for packet switching either at the local switch or toll switch levels. Circuit switching calls are controlled to be connected to a circuit switching system and packet switching calls are controlled to be connected to a packet switching system separately.
The method disclosed in U.S. Pat. No. 4,486,878 is simpler in the structure of the networks and is most preferable in service, when viewed from ISDN subscriber side. The method may therefore be the most popular solution when ISDN prevails widely in future. But then, all of the prior switching systems should be invalidated for ISDN subscribers. Thus, an immediate application of the patent method may not be very economical.
If the separate switching method is adopted, the switching section of a prior digital switching system can be still used as the switching section for circuit switching call. In order to promote the introduction of ISDN, the separate switching method seems to be a more feasible method, at least during the initial stage in the ISDN introduction process.
If the latter method should be adopted, however, a problem still remains unsolved; namely, at which location within a network should the switching system dedicated to the packet switching be placed. It is desirable that the location be decided upon depending on the ratio of the traffic between packet switching calls and circuit switching calls. More specifically, if the traffic ratio is high in the packet switching calls, the switching system should be installed at a local switch. If the traffic ratio is high in the circuit switching calls, it should be installed at a toll switch. Generally, it is expected that, in the initial stage of implementation of ISDN, the traffic of packet calls is much smaller than that of circuit switching calls. Thus, it is considered economically more practical for the packet switching system to be installed at a toll switch and subscribers should have access to it whenever necessary.
In order to operate such a toll connecting trunk most effectively in view of economical considerations, it is desirable that the tool connecting trunk can be shared in use by circuit switching calls and packet switching calls. The ratio in the number between channels used by packet switching calls and those used by circuit switching calls on the toll connecting trunk should be variable or, in other words, should be a variable boundary in structure. The ratio may be semi-variable, but is preferred to be dynamically variable to achieve a higher effect. The existing systems cannot meet such requirements.